Biological systems of cells and cytoskeletal elements can form a nematic phase where elongated fibres align parallel to each other, inducing partial orientational order. This order is described by a macroscopic director field that reflects the local orientation of the system. Topological defects in the nematic order are singularities in the director field; they are quite common and classified by their topological charge.

The emergence of nematic order from an initially isotropic, or disordered, organization of the fibres on deformable closed surfaces plays a pivotal role in the morphogenesis of active biological matter. A notable example is the regeneration of Hydra, a freshwater basal invertebrate: an excised fragment first folds into a spherical shell in an approximately isotropic state, and nematic order subsequently develops in the actin fibres of the epithelial tissue. On a surface with the topology of a sphere, the formation of topological defects is unavoidable, with a global constraint requiring the total topological charge to equal +2.

In this talk, I present a continuum model that couples the two-dimensional Landau-de Gennes order tensor, which describes the in-plane nematic ordering, with the mechanics of a mass-conserving, deformable spherical shell. By investigating the isotropic-to-nematic phase transition driven by a reduction in temperature, mimicking the natural induction of nematic order in actomyosin fibres, we perform both linear and weakly non-linear bifurcation analyses. We demonstrate that the onset of nematic ordering spontaneously breaks spherical symmetry, yielding distinct equilibrium morphologies governed by the shell’s deformability. Axisymmetric configurations, characterised by two +1 topological defects at the poles, emerge via a discontinuous (transcritical) bifurcation, resulting in a globally stable prolate shape with meridional director alignment, alongside a metastable oblate shape. Conversely, non-axisymmetric configurations, featuring four +1/2 defects arranged in a squared geometry, arise via a continuous (supercritical) bifurcation. We reveal that shell softness drives the first-order nature of the transition; in the limit of infinite stiffness, all bifurcations become continuous. Furthermore, our analysis indicates that integer defects strongly couple with local mass redistribution, manifesting as shell thinning or thickening, whilst half-integer defects induce no such local deformation. These findings provide a purely mechanical framework for understanding the formation of the body axis, which determines the future positions of head and foot, as well as defect-mediated morphogenesis in biological vesicles.